This invention relates to drain valves and, more particularly, to drain valves used in aircraft fuel tanks. However, the valves could also be used for other purposes.
Drain valves are often provided on aircraft fuel tanks so that liquid within the tank, such as water formed from condensated moisture, may be readily removed or drained. Such drain valves typically include a drain port or opening through which the liquid in the tank is drained and a valve member or poppet which moves axially in relation to the port in order to open and close the port. In operation, a preferred form of drain valve has a poppet which is biased to close the port and can be simply pushed inwardly of the fuel tank by use of a screwdriver or other simple tool to open the port. Upon release, the poppet automatically returns to its part-closing position under spring pressure. Examples of such drain valves are shown in Koller, U.S. Pat. No. 3,703,189, and Lipman, U.S. Pat. No. 2,655,932. Another and improved construction is disclosed in Koller et al. U.S. Pat. No. 4,351,355, the disclosure of which is hereby incorporated by reference herein. In the Koller et al. '355 patent, a retainer assembly 52 comprising a pair of interfitting retainer members 58 and 60 has slots for receiving the ends of a pin 40 extending through a valve stem 38 of a poppet or valve member 26. The retainer assembly further has surfaces for supporting the pin 40 and, in turn, the valve member 26 when the valve is either opened or closed. In addition, by rotation of the valve member 26, the pin 40 may be aligned with a slot in one of the retainer members 60 which permits of lowering of the valve member 26 to expose an O-ring sealing member 32 for inspection or replacement. When so lowered, the valve member 26 has a conical valve surface 96 which contacts a beveled valve surface 98 formed on the upper end of the port 20 to provide a temporary seal while the O-ring seal 32 is being inspected or replaced.
It has been recognized that drain valves are subject to being opened as a result of lightning strikes. U.S. Pat. No. 4,437,487 granted March 20, 1984 to Frank E. Marmon mentions that drain valves have been provided with special locks or locking devices to secure the valve in a closed position to protect the valves from lightning strikes and that these require the positive act of a service person. The Marmon '487 patent also discusses the desirability of push-type drain valves which are protected from opening in the event of lightning strikes and proposes a rather complex construction wherein the "blast pressure" of the lightning is allowed to pass through a passage in a piston to allow a closing force to be exerted on the surfaces that close the valve which is greater than the opening force due to a difference in area between the "blast pressure" tending to open the valve and the "blast pressure" tending to close the valve.
Applicant is not informed as to whether or not the drain valve disclosed in the above-mentioned Marmon '487 patent is successfully protected against opening in the event of a lightning strike. Whether it is or not, the need exists for a simple, inexpensive drain valve construction having substantially the advantages of the aforementioned Koller et al. '355 patent which, in addition, is protected against opening in the event of a lightning strike.